a(n) = Sum_{d|n} 8^(d-1).

Terms

a(0) =1a(1) =9a(2) =65a(3) =521a(4) =4097a(5) =32841a(6) =262145a(7) =2097673a(8) =16777281a(9) =134221833a(10) =1073741825a(11) =8589967945a(12) =68719476737a(13) =549756076041